(1) Isolate, identify and determine the state of maturation of thyminsensitive pre-T cells and to establish the mechanism through which thymin acts to convert these cells to cells resembling intra-thymic T cells. The effect of thymin treatment in vitro on subsequent immunologic reactivity of these cells shall also be studied. (2) Develop a sensative quantitative assay for thymin and thymin-sensitive cells. Subsequently an effort shall be made to extend these observations to human materials.